The present invention relates to an image processing method and apparatus for reproducing a color image according to hue-based color signals which are obtained by converting optical image information into electric signals. More particularly, the invention relates to an image processing method and apparatus for correcting color mismatching between an imaged object and the reproduced image by means of color matching.
A conventional image processing method will be explained with reference to the example of a copying machine. In such an apparatus, a color image is reproduced on a sensitized material by scanning an original document to be copied with a color image sensor to produce color signals on a hue basis, the resulting signals are digitized, and then light-generating elements such as LEDs are driven by the color signals and a sensitized recording material is scanned with the resulting light signals to record an image thereon.
However, since the spectral absorption characteristics of the original being copied and the recording materials generally do not correspond one-to-one, it is necessary to correct the color signals using a color matching technique before the image is reproduced.
Conventional image processors employing color matching have a construction as shown in FIG. 1. Circuits 1, 2, 3 in FIG. 1 are logarithmic conversion circuits, which, for example, in a copying machine are provided for respective color signals R, G, B produced by scanning the document to be copied with an image sensor and converting the resulting signals into digital values via an A/D converter. The circuits 1, 2, 3 produce respective logarithmic values C, M, Y corresponding to the respective color signals R, G, B as complementary color signals. This logarithmic process is employed partly because of the logarithmic characteristic of human vision. In order to minimize the size of the correction circuits, they are implemented in so-called "firmware" by pre-storing outputs corresponding to inputs in a read-only-memory (ROM).
Circuits 4, 5, 6 are color correction circuits, which perform correcting operations upon the respective color signals in such a manner that the hues of the reproduced image are made to accurately correspond with those of the document being copied. This correcting operation is performed in accordance with the operations indicated by the following equation (1). That is, as indicated by equation (1), cyan-related data C, magenta-related data M, and yellow-related data Y are added after being multiplied by predetermined conversion coefficients a.sub.11 to a.sub.33. EQU C'=a.sub.11 .multidot.C+a.sub.12 .multidot.M+a.sub.13 .multidot.Y EQU M'=a.sub.21 .multidot.C+a.sub.22 .multidot.M+a.sub.23 .multidot.Y (1) EQU Y'=a.sub.31 .multidot.C+a.sub.32 .multidot.M+a.sub.33 .multidot.Y
Expressing the above equation in the form of a matrix operation, ##EQU1##
The conversion coefficients a.sub.11 to a.sub.33 are derived empirically and pre-stored in ROMs. During actual correction operations, the above operations indicated by equation (1) are performed by reading the conversion coefficients using, for example, a microprocessor.
Circuits 7, 8 and 9 are output adjusting circuits which make fine adjustments for the level of the corrected color signals C', M', Y' produced by the respective color correction circuits 4, 5, 6 before the signals are applied to LEDs to expose the recording material to light based on the finely adjusted color signals C.sub.O, M.sub.O, Y.sub.O.
The above technique is applicable not only to copying machines but also to correcting color mismatch between a hard copy and its image displayed on a television monitor.
However, in the conventional picture processing method, a common data length is employed for all the data of transformation coefficients stored in ROM, and hence usually a large storage capacity and large size of operating circuit are necessary.
In the case of linear matrix transformation, the storage capacity and the size of the required operating circuit are not too serious problems owing to the relatively small number of data values required for the transformation coefficient. However, the matrix operation described above with respect to equations (1) and (2) applies only to the case of linear correction where the conversion coefficients a.sub.11 to a.sub.33 are constant. On the other hand, for nonlinear matrix transformation and direct color matching in which the output signal must be made to accurately correspond with the input color signal, the required ROM storage capacity and the size of the operating circuit become tremendously large because the matrix of transformation coefficients is complex and data values for many conditions are needed.